1. Field of the Invention
The present invention relates to a wireless (or radio) communications system and, more particularly, to a transmission power control method and device in the wireless communications system.
2. Description of the Related Art
In a W-CDMA (Wide band-Code Division Multiple Access) system, which is a typical wireless communications system, HSDPA (High Speed Downlink Packet Access), which is a high-speed packet transmission system for downlink, and EUDCH (Enhanced Uplink Dedicated Channel), which is a high-speed packet transmission system for uplink, have been standardized. According to these packet transmission systems, a base station (Node B) carries out scheduling and, only when data transmission/reception is performed, allocates wireless resources to a plurality of mobile stations (VEs) according to time multiplexing or code multiplexing, resulting in the enhanced efficiency of use of the wireless resources. This W-CDMA system will be described briefly with reference to the accompanying drawings.
FIG. 1 schematically shows a mobile communications network as an example of the W-CDMA system. Here, a plurality of base stations 11 and 12 are connected to a base station controller 10, and the base station controller 10 can be further connected to an external network (not shown). Each of the plurality of base stations can accommodate a plurality of mobile stations. Here, it is assumed that mobile stations 21 and 22 are connecting to the base station 11, a mobile station 24 is connecting to the base station 12, and a mobile station 23 is in soft handover (hereinafter, referred to as SHO where deemed appropriate) and therefore is connecting to both the base stations 11 and 12.
Each of the mobile stations 21 to 24 always uses a dedicated channel (DPCCH: Dedicated Physical Control Channel) for transmission/reception to/from its corresponding base station. Further, the mobile stations 22 and 24 are performing data reception using HSDPA, and the mobile stations 23 and 24 are performing data transmission using EUDCH. The mobile stations 22 and 24, which are performing data reception using HSDPA, use HS-SCCH (High Speed-Shared Control Channel) and HS-PDSCH (High Speed-Physical Downlink Shared Channel) for reception and HS-DPCCH (High Speed-Dedicated Physical Control Channel) for transmission. The mobile stations 23 and 24, which are performing data transmission by using EUDCH, use E-HICH (Enhanced-Hybrid ARQ Indicator Channel), E-AGCH (Enhanced-Absolute Grant Channel), and E-RGCH (Enhanced-Relative Grant Channel) for reception and E-DPCCH (Enhanced-Dedicated Physical Control Channel) and E-DPDCH (Enhanced-Dedicated Physical Data Channel) for transmission. In other words, a mobile station that is executing HSDPA or EUDCH, always uses a channel called a dedicated channel for transmission and reception, apart from channels for data transmission and reception. The dedicated channel is used to transmit a pilot signal, which is used for channel estimation in order for mobile and base stations to secure synchronization and carry out demodulation, and a TPC (Transmission Power Control) signal, which is a control signal for closed-loop power control (inner loop power control).
Closed-loop power control is performed on the transmission power for a dedicated channel so that the quality of the dedicated channel becomes closer to target quality (here, target SIR (Signal to Interference Ratio)). For example, in the transmission power control of an uplink dedicated channel, a base station compares the SIR of the dedicated channel actually received from a mobile station in question with a target SIR set by the base station controller 10. If the reception SIR actually received is smaller than the target SIR, the base station transmits a TPC signal instructing that the transmission power be increased, through a downlink dedicated channel. Otherwise, the base station transmits a TPC signal instructing that the transmission power be reduced. The mobile station increases or reduces the power for the dedicated channel in accordance with the instruction of the TPC signal received through the dedicated channel.
Here, in the case where a mobile station uses dedicated channels for transmission/reception to/from a plurality of base stations, that is, where a mobile station is in a soft handover (SHO) state like the mobile station 23, the mobile station receives a plurality of TPC signals. Among the received TPC signals, if the mobile station receives at least one TPC signal giving an instruction to reduce the power, the mobile station is controlled to reduce its transmission power. This is because during SHO, communications can be carried out as long as any one of the plurality of base stations meets desired quality, and because increasing transmission power to make all the base stations receive sufficient quality leads to increased interference with another user, which is not favorable.
In addition, the physical layer of a base station generates Out-of-sync when the reception quality of a dedicated channel has deteriorated below a predetermined level. Hereinafter, generation of Out-of-sync and notification of RL_Failure for indicating an out-of-sync state will be described concretely based on the description of 3GPP TS25.214 v6.6.0 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD); (Release 6).
FIG. 2A is a time chart for describing a process of RL_Failure generation in a base station (Node B), and FIG. 2B is a time chart showing changes in received quality, for describing a conventional transmission power control method. Referring to FIG. 2A, in a normal-mode, when the target SIR of a dedicated channel is set at a value (SIR_normal) notified from the base station controller 10, a reception SIR (dotted line) of a mobile station (UE: User Equipment) in question is changed to be closer to the target SIR through the above-described closed-loop power control.
In this closed-loop power control, if the reception SIR of this mobile station deteriorates and falls below a sync-securing quality threshold SIR_th (dashed-and-dotted line), which is set by the base station on its own, a sync detection section starts a timer and, when a predetermined period of time T_timer1 [ms] has elapsed, notifies Out-of-sync to a sync state management section. When the sync state management section is notified of Out-of-sync a predetermined number of times (N_OUTSYNC_IND), the sync state management section starts a timer. If the sync state management section does not receive In-sync notifying that the reception SIR of the dedicated channel is at the predetermined level or above before the timer exceeds a predetermined period of time (T_RLFAILURE), the sync state management section transmits a signal RL_Failure notifying of an out-of-sync state to the base station controller 10. Note that the predetermined number of times N_OUTSYNC_IND and the timer's predetermined period T_RLRAILURE are values determined by the base station controller 10 and notified to each base station beforehand. When the base station controller 10 receives RL_Failure from the base station, the base station controller 10 determines that there is a problem in a propagation path between the base station and mobile station in question or in the mobile station itself, and carries out a countermeasure such as disconnecting the dedicated channel in connection.
In such control of a dedicated channel, a state such that a mobile station is carrying out the transmission/reception of a dedicated channel is referred to as a Cell_DCH state. On the other hand, when no data transmission/reception is carried out for a long time, the mobile station changes to a state other than the Cell_DCH state, for example, a Cell_FACH state. In the Cell_FACH state, the mobile station only receives a predetermined downlink channel at predetermined time intervals and carries out no transmission/reception of a dedicated channel. Therefore, for the mobile station in the Cell_FACH state, uplink synchronization is not established.
When the mobile station in such a Cell_EACH state starts transmitting/receiving a predetermined amount of data or more, the mobile station changes to the Cell_DCH state. However, a considerable delay may be created by the time that elapses before the state change is finished and then data transmission/reception is started. The reason is as follows: a dedicated channel resource needs to be allocated to the mobile station through the base station controller 10; predetermined control parameters need to be notified to the mobile station and base station; and uplink synchronization needs to be established. Therefore, if a change from the cell_DCH state to the cell_FACH state or vice versa occurs frequently, a data transmission delay increases, resulting in the degraded quality of service to a user. In addition, the processing load on the base station controller 10 is increased, and the number of control signals exchanged between the base station controller 10 and the base station is increased, which are not favorable.
To improve such circumstances, 3GPP (3rd Generation Partnership Project) defining the W-CDMA specifications is conducting studies about allowing a mobile station that has not performed data transmission for a long time also to maintain the Cell_DCH state and thus allowing data transmission/reception to be started without a state-change delay. In this case, however, as mentioned above, since the number of mobile stations that transmit dedicated channels increases by a large amount, the apprehension arises that uplink interferences are increased and the system capacity is reduced.
Therefore, there have been proposed several methods to reduce the power of an uplink dedicated channel when no data transmission/reception is being carried out (see 3GPP TR25.903 v0.2.0 (2005-11) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Continuous Connectivity for Packet Data Users; (Release 7)). For example, proposed is a method of changing a target SIR depending on whether or not data transmission/reception is being carried out. Specifically, as shown in FIG. 26, when data transmission/reception has not been carried out for a certain period of time, a base station that is conducting scheduling (serving base station) decreases the target SIR to a predetermined reduced quality level (SIR_reduced) capable of TPC detection. Because of this decrease in the target SIR, the transmission power of the mobile station for the dedicated channel is also reduced through the closed-loop power control. As a result, it is possible to effectively reduce the amount of uplink interference this mobile station might exert on another mobile station. Such a state of a mobile station whose transmission power for a dedicated channel is reduced by decreasing the target SIR as described above is referred to as reduced power mode, idle traffic mode, or the like. In addition, the state of a mobile station that maintains the Cell_DCH state and is not in the reduced power mode is referred to as normal mode, active mode, or the like.
Additionally, the above-mentioned modes are determined depending on the relative magnitude of transmission power. Therefore, reversely, the normal mode can be called increased power mode, and the reduced power mode can be called normal mode.
Incidentally, a serving base station is generally determined by a base station controller connected to base stations. As an example of the method of determining the serving base station, a mobile station first measures the reception quality of a pilot signal from each base station and notifies the measurement results, through the respective base stations, to the base station controller connected to the base stations. The base station controller estimates a path loss for each base station based on the reception quality of each pilot signal and determines, as the serving base station, a base station whose estimated path loss is small. This determination result is notified to each base station.
However, when the reception quality of a dedicated channel has deteriorated below a predetermined level, the sync detection section in a base station sends Out-of-sync to the sync state management section to notify that the quality of the dedicated channel has deteriorated and sync precision has declined (or is likely to decline). During the above-described reduced power mode, since the reception SIR of the dedicated channel decreases, the reception SIR becomes equal to or lower than the predetermined deterioration threshold SIR_th set in the sync detection section, leading to the possibility that Out-of-sync is generated frequently.
Moreover, the sync-securing quality threshold SIR_th, which is a criterion for sync determination used by the sync detection section, is a value preset by a base station on its own, and therefore there is no guarantee that all base stations have the same sync-securing quality threshold SIR_th. In such an environment, when a serving base station that is being used by a mobile station decreases the target SIR by a predetermined value, a reduction occurs in the transmission power of a dedicated channel from the mobile station in the reduced power mode. For another base station (non-serving base station), there might be a possibility that the reception SIR of its corresponding dedicated channel deteriorates below the sync-securing quality threshold SIR_th of this non-serving base station.
As described above, if the reception SIR becomes equal to or lower than the predetermined deterioration threshold SIR_th and thereby Out-of-sync is generated frequently, then the sync state management section transmits to the base station controller RL_Failure notifying that the wireless link with the mobile station is likely to go out of sync, and thereby the base station controller disconnects (releases) the dedicated channel between the base station in question and the mobile station. In the case where the base station controller is connected to a plurality of base stations through a network, if the notifications of RL_Failure are frequent, the problem arises that the number of control signals exchanged between the base station controller and the base stations is increased, resulting in the increased load on the network.
If a dedicated channel is disconnected, it is needed to carry out again the allocation of a dedicated channel resource, notification of predetermined control parameters, establishing of uplink synchronization, and the like when transmission data occurs. Therefore, data transmission/reception cannot be performed immediately.
For a non-serving base station during soft handover in particular, it is more serious that RL_Failure is generated frequently. In general, since a path loss occurring between a mobile station and each base station during soft handover varies from one base station to another, the reception quality of the uplink each base station receives also varies. In addition, a base station providing a better propagation environment, that is, a base station having a smaller path loss is generally selected as a serving base station, because the serving base station transmits control signals related to scheduling. Therefore, in many cases, a non-serving base station principally receives lower reception quality than a serving base station. Accordingly, if the serving base station decreases the target SIR during the reduced power mode, the non-serving base station will receive even lower reception quality and hence have a higher possibility of going out of sync than the serving base station.